Ruby Toolbox - Explore and compare open source Ruby libraries
GitHub Visualizer - Enter user/repo and see the project visually
AlternativeTo - AlternativeTo lets you find apps and software for Windows, Mac, Linux, iPhone, iPad, Android, Android Tablets, Web Apps, Online, Windows Tablets and more by recommending alternatives to apps you already know.
SyntaxDB - Easily look up programming syntax for multiple languages
LibHunt - LibHunt tracks mentions of software libraries on relevant social networks. Based on that data, you can find the most popular projects and their alternatives.
Gource - Gource is a software version control visualization tool.